The R-Process Alliance: Discovery of the First Metal-poor Star with a Combined r- and s-process Element Signature

• Published in: The Astrophysical journal 2018-08, Vol.862 (2), p.174
• Main Authors: Gull, Maude, Frebel, Anna, Cain, Madelyn G., Placco, Vinicius M., Ji, Alexander P., Abate, Carlo, Ezzeddine, Rana, Karakas, Amanda I., Hansen, Terese T., Sakari, Charli, Holmbeck, Erika M., Santucci, Rafael M., Casey, Andrew R., Beers, Timothy C.
• Format: Article
• Language: English
• Subjects: Abundance; Astrophysics; Carbon; Dwarf galaxies; Giant stars; Iron; Mass transfer; Metallicity; Neutron stars; Nuclear capture; Nuclear fusion; Organic chemistry; Red giant stars; Signal processing; Signal to noise ratio; Star mergers; Stars & galaxies; Thorium
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/aacbc3

We present a high-resolution ( R  ∼ 35,000), high signal-to-noise ratio (S/N > 200) Magellan/MIKE spectrum of the star RAVE J094921.8−161722, a bright ( V  = 11.3) metal-poor red giant star with [Fe/H] = −2.2, identified as a carbon-enhanced metal-poor (CEMP) star from the RAVE survey. We report its detailed chemical abundance signature of light fusion elements and heavy neutron-capture elements. We find J0949−1617 to be a CEMP star with s -process enhancement that must have formed from gas enriched by a prior r -process event. Light neutron-capture elements follow a low-metallicity s -process pattern, while the heavier neutron-capture elements above Eu follow an r -process pattern. The Pb abundance is high, in line with an s -process origin. Thorium is also detected, as expected from an r -process origin, as Th is not produced in the s -process. We employ nucleosynthesis model predictions that take an initial r -process enhancement into account, and then determine the mass transfer of carbon and s -process material from a putative more massive companion onto the observed star. The resulting abundances agree well with the observed pattern. We conclude that J0949−1617 is the first bonafide CEMP- r  +  s star identified. This class of objects has previously been suggested to explain stars with neutron-capture element patterns that originate from neither the r - nor the s -process alone. We speculate that J0949−1617 formed in an environment similar to those of ultra-faint dwarf galaxies like Tucana III and Reticulum II, which were enriched in r -process elements by one or multiple neutron star mergers at the earliest times.